1. Field of the Invention
The present invention relates to an automatic speed reducing ratio-switching apparatus, which is capable of automatically switching a speed-reducing ratio transmitted from an output shaft, for example, to another member or an apparatus using a planetary gear mechanism when an applied load exceeds a predetermined torque. As described later, the present invention can be applied and utilized extensively in a variety of different fields.
2. Description of the Related Art
A speed reducing ratio-switching mechanism has been hitherto applied, for example, to machine systems such as construction machines. In such machine systems, an electric cylinder is used as an actuator in an expansion/contraction operation system for driving a link mechanism.
In such an electric cylinder, a rotary shaft is connected to an input portion of an electric motor in a casing, and a screw shaft is arranged within the rotary shaft. The screw shaft is screw-engaged with a nut member, which is rotatably supported inside the casing. Two pairs of planetary gear mechanisms, which have different speed reducing ratios, are provided between the rotary shaft and the nut member. Each of the planetary gear mechanisms comprises a sun gear together with planet gears, which are meshed with each of the sun gear and an internal gear provided inside the cylindrical casing and which perform planetary motion. Each of the sun gears connects to the rotary shaft via a one-way clutch, in which the direction of engagement is different, comprising a forward rotation direction and a reverse rotation direction. A planetary support shaft, which rotatably supports the planet gears of each of the planetary gear mechanisms, is connected to the nut member.
In the electric cylinder, when the electric motor is driven and rotated in a forward rotation direction, the rotary shaft is also rotated in the forward rotation direction, whereas the nut member is rotated in the forward rotation direction through the aid of the planetary gear mechanism, which has a small speed reducing ratio, and the screw shaft is operated and moved in an expandable manner. On the other hand, when the electric motor is driven and rotated in a reverse rotation direction, then the rotary shaft is also rotated in the reverse rotation direction, whereas the nut member is rotated in the reverse rotation direction through the aid of the planetary gear mechanism, which has a large speed reducing ratio, and the screw shaft is operated and moved in a contractible manner (see Japanese Laid-Open Patent Publication No. 2003-184982).
However, in the electric cylinder described above, it is necessary to use two types of planetary gear mechanisms, having different speed reducing ratios, in order for the screw shaft to perform expanding and contracting operations. In this case, the number of parts is increased, and the entire electric cylinder is increased in size. When using the planetary gear mechanism of the electric cylinder, as described above, an expanding operation is performed at low speed with a large thrust force, while a contracting operation is performed at high speed with a small thrust force, irrespective of the magnitude of load torque applied to the electric cylinder. Therefore, the movement speed of the screw shaft cannot be increased and performed at a high speed during the expanding operation, even if the load torque applied to the electric cylinder is small.
In view of the above, the present applicant has suggested an automatic speed reducing ratio-switching apparatus, in which it is possible to control torque, and thus to transmit torque at a high speed by automatically switching the speed reducing ratio in response to operation of a displacement member included in the actuator (Japanese Laid-Open Patent Publication No. 2006-22950).